This invention relates to a low temperature air fractionation process and apparatus therefor.
There are many processes in the prior art based on a system wherein air is compressed, prepurified, cooled, and roughly fractionated into a nitrogen-rich fraction and an oxygen-rich liquid in a high pressure stage of a two-stage rectification unit. The two fractions are introduced at least in part to a low pressure stage of the rectification unit and further separated into oxygen and nitrogen. In U.S. Pat. No. 2,666,303, at least one gaseous nitrogen fraction is removed, heated, and engine-expanded at least in part. In this process, nitrogen from the low pressure stage is expanded to produce cold, i.e., refrigeration values. This is more economical than a cooling cycle operated with nitrogen from the high pressure stage, assuming that the low pressure stage is operated under a pressure, higher than atmospheric pressure which is the usual case. The conventional process exhibits the drawback, however, that the expanded portion of the gaseous nitrogen fraction from the low pressure stage can no longer be further utilized for purposes where the gaseous nitrogen must be under an elevated pressure.